1. Field of the Invention
The present invention relates to a fuel cell system for generating electricity using fuel gas including hydrogen, and particularly relates to supplying of fuel gas in a fuel cell system.
2. Related Art
In recent years, there has been great expectation for fuel cells, as means for suppressing global warming and environmental destruction, and also as next-generation electrical-generating systems, and research is being actively pursued. Fuel cells generate energy by electrochemical reaction between hydrogen and oxygen, examples thereof including phosphoric-acid fuel cells, molten carbonate fuel cells, solid-oxide fuel cells, polymer electrolyte fuel cells, and so forth. Of these, polymer electrolyte fuel cells can be activated from room temperature and furthermore are small and have high power, and accordingly are gathering attention as a power source for vehicles (motorcycles, automobiles) and for portable power sources and the like.
With general polymer electrolyte fuel cells, an electrode structure, which is a basic configuration unit of the fuel cell, is sandwiched between separators, thereby forming a single cell. A combination thereof of the order of tens to hundreds of cells is used as a stack. An electrode structure serving as a basic configuration unit of a stack is formed of two electrodes, which are an anode electrode (fuel electrode) and cathode electrode (air electrode), and a polyelectrolyte membrane introduced therebetween. Also, normally, these electrodes are formed from a catalyst layer at which an oxidation and reduction reaction occurs in contact with the polyelectrolyte membrane, and a gas dispersion layer in contact with the catalyst layer.
Polymer electrolyte fuel cells having such a configuration generate electricity by supplying fuel including hydrogen to the anode electrode side and supplying oxygen or air to the cathode electrode side. Upon supplying fuel gas to the anode electrode, the hydrogen included in the fuel gas is ionized on the electrode catalyst. The ionized hydrogen moves to the cathode electrode side through the electrolytic membrane. On the other hand, the electrons generated in the aforementioned ionization step are extracted to an external circuit, thereby providing DC electric energy.
The above-described fuel cells also generate water as well as electricity during generating of electricity by the electrochemical reaction between the hydrogen gas serving as the fuel gas, and the oxygen. Normally, this water is purged from channels at both electrodes due to a gas flow supplied at a sufficient gas flow speed. However, in some cases, the gas flow speed drops due to a low load or the like. Such a case can lead to an undesired phenomenon in which the water therewithin is not completely discharged and closes off the channel (hereafter referred to as “flooding”). There has been a problem in that electricity generation becomes unstable when the channel of the fuel gas is closed off due to this flooding.
Furthermore, a method is being used wherein unreacted fuel gas discharged from the fuel cell is re-circulated to improve system efficiency. However, with such a method, vapor is also included in the re-circulated gas, so there has been a problem in that flooding occurs even more easily.
In view of such problems, an invention has been disclosed which solves flooding by external discharge of fuel gas (e.g., Japanese (Unexamined) Patent Application Publication No. 2002-237322). The invention disclosed in this publication solves flooding by closing a valve within the circulation system channel and opens a discharge valve to prevent backflow and to increase the flow speed within the fuel cell channel, in the event that flooding occurs due to reduction in suction of an ejector pump or the like.
Moreover, an invention has been disclosed wherein flooding is solved using a compressor such as an electric pump or the like in the circulation of the fuel gas (e.g., Japanese (Unexamined) Patent Application Publication No. 2002-280027). That is to say, while, in general, the fuel gas circulation amount is determined corresponding to the load of the fuel gas, the invention disclosed in this publication provides an arrangement in which the circulation flow is corrected corresponding to the electricity generating state. That is to say, in the event that flooding or the like occurs, the output of the pump is increased to enhance the circulation flow, thereby solving flooding while reducing external discharge of fuel gas.
Furthermore, an invention has been disclosed wherein hydrogen off-gas is extracted, the hydrogen collected by a hydrogen collector is stored in a storage tank, and then supplied to the fuel cell again using a supply pump (e.g., Japanese (Unexamined) Patent Application Publication No. 62-272468). Moreover, an invention has been disclosed wherein hydrogen off-gas is extracted, and hydrogen collected by a hydrogen generator is stored in a hydrogen storage unit and then supplied to the fuel cell again (e.g., Japanese (Unexamined) Patent Application Publication No. 5-13094). The inventions disclosed in these publications suppress consumption of fuel in the fuel cell system by re-circulating the unreacted gas discharged from the fuel cell.
However, with the invention disclosed in Japanese (Unexamined) Patent Application Publication No. 2002-237322, fuel consumption efficiency decreases due to discharging the fuel gas externally in order to increase the flow speed within the fuel cell channel, thereby deteriorating the efficiency of the overall system. Also, while the invention disclosed in Japanese (Unexamined) Patent Application Publication No. 2002-280027 is capable of reducing the external discharge amount of fuel gas, a compressor such as an electric pump or the like which is capable of blowing out water droplets in the channel of the fuel cell, becomes necessary, which leads to the problem of increased weight and size. Further, energy necessary for driving this compressor must be supplied, meaning that the overall efficiency of the system is deteriorated to that extent.
Also, with the inventions disclosed in Japanese (Unexamined) Patent Application Publication Nos. 62-272468 and 5-13094, the hydrogen discharged from the fuel cell can be efficiently used up, but the flow speed within the reactant gas channel cannot be increased, leading to the problem that flooding easily occurs.